Conference calls are a popular means for having a meeting among a geographically diverse group of participants without having to expend the time and money necessary to meet in person. A conference call is a telephonic connection among a number of participants in diverse locations communicating on full-duplex circuits. A telephone system that can provide conference calls requires special hardware, called a "conference bridge."
FIG. 1 shows a typical conference bridge of the prior art. For purposes of describing the prior art, a digital conference bridge of the type used in long-distance service is shown. Such conference bridges operate in an environment where voice signals are digitally encoded into a 64 Kbps data stream. Conference bridge 10 interconnects a plurality of participants 1-N to each other, so that each participant may speak and be heard by all of the other participants, and may hear all of the other participants. To this end, conference bridge 10 has a plurality of inputs 12 and a plurality of outputs 14, one of each for each participant. For the purpose of illustrating FIG. 1, inputs 12 are shown on the left side of the drawing and outputs 14 am on the right. In reality, an input and output pair go to each of the participants.
Each input 12 is connected to a speech detector 16 which detects speech on the input by sampling the 64 Kbps data stream and determining the amount of energy present over a given time. If the energy exceeds a predetermined threshold, and is greater than the worst case estimated echo, then it is presumed that the signal contains speech. Each speech detector 16 controls a switch 18, so that switch 18 is closed when speech is detected, thus allowing the speech to pass. If the energy in the data stream is not above the predetermined threshold, then the signal is presumed to contain background noise. When no speech is detected, switch 18 is open, which reduces unwanted background noise from non-speakers, and thus increases the clarity of the speech signal delivered to the participants. An automatic gain control (AGC) device 19 is connected between speech detector 16 tap and switch 18, in order to provide a normalization of the volume (gain) of speech across all inputs 12. Speech detector 16 provides AGC 19 with a positive or negative value, which is added by AGC 19 to the signal on input 12.
Speech signals that pass through switch 18 are then mixed at summing amplifier 20, which sums the various speech signals and amplifies them for distribution. The combined signals are delivered to distributor 22, which sends the combined signals to all outputs 14. The combined signals are received at subtractors 24, which subtracts the incoming speech (if any) from the specific participant, in order to prevent that participant from hearing echo of his own voice on the line.
A further source of echo, which is well known in the art, is echo of a speech signal reflecting from a hybrid at the far end. In a conference bridge of FIG. 1, such echo is a major problem, because echo is coming in on N inputs 12, is amplified at AGC 19 and summing amplifier 20, and is then sent back to all participants on outputs 14. Thus, any echo is amplified N-1 times, creating intolerable noise levels. As a result, a further echo canceler circuit 26 is added to each input/output pair, with additional circuitry to reduce echo. Echo canceler 26 taps output 14 after subtractor 24 (so that any previous source of echo has been eliminated) and feeds the speech signal to a canceling circuit 28. Canceling circuit 28 estimates the amount and the time delay of echo and causes this amount to be subtracted from the signal on input 12 at subtractor 30. There is one echo canceler 26 per input/output pair, even though only one is shown (on input/output N) for clarity.
Conference bridges of this type work well for standard digital conference calls. There is, however, a new method for transporting telecommunications, which is to packetize a digital speech signal and transport the packets. One example of such packetization is asynchronous transfer mode (ATM). ATM cells encapsulate digitized speech and are sent at a high rate of speed over a self-routing ATM network. An obvious solution to the problem of providing a conference bridge for an ATM communications network is to remove the communications data from each ATM cell at an ATM-to-network converter, shown in phantom at 32 on inputs 12 (FIG. 1), perform the conferencing function as before, and re-encapsulate the communications data at a network-to-ATM converter, shown in phantom at 34 on outputs 14. Such a system would inevitably be slow and cumbersome, since every time communications form is converted, the communication is slowed due to packetization delay.
A problem in the art is that there is no conference bridge in the prior art that can support packetized speech signal transmission without conversion of the packet stream, causing packet delay.